1. Field of the Invention
The field of the invention relates to electrical ground fault protection generally, and more particularly to certain new and useful advances in devices and methods for ground fault detection and interruption of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Description of Related Art
A ground fault is an undesirable condition in an electrical system, in which electrical current flows to the ground. A ground fault happens when the electrical current in a circuit leaks outside of its intended flow path. Ground fault circuit interrupters (GFCIs) or residual current devices (RCDs) are designed to protect such electrical hazards by interrupting a circuit when there is a ground fault, or residual current, such as a leakage of electrical current from an energized line conductor to ground. Conventional electric circuits normally carry balanced electrical currents, with the return current from an electrical load flowing through a neutral conductor. If no ground fault current is flowing, the phase and neutral currents of a branch circuit will sum to zero. In the event of a ground fault, the phase and neutral currents do not sum to zero and the difference between the phase and neutral currents is the ground fault current.
In some instances, the ground fault may result in lethal shocks or electrocution, such as in the case of a person who makes inadvertent contact with the phase conductor. The U.S. National Electrical Code (NEC) requires ground fault protection for both electric shock and equipment protection. A conventional GFCI is designed to detect predetermined levels (typically in the range of 4 to 6 milliamperes) of differential currents and to open the circuit, for example by tripping a circuit breaker, to remove the indicated shock hazard. If a differential current below the predetermined level is detected, current is normally allowed to flow uninterrupted. In some instances, ground fault protection devices alternatively provide a warning of a ground fault hazard but do not interrupt current flowing in the affected circuit, for example in fire protection applications where the ground current hazard is considered less dangerous than interrupting the current to fire prevention equipment.
FIG. 1 shows a conventional electrical circuit 10 such as a branch circuit having an AC power source 11 providing electrical power to a load 17. The load 17 is housed in an enclosure 18 which is connected to ground via a conductor 19. A conventional GFCI device 12 having switching contacts 13a, 13b is configured to measure the current balance between a line or phase conductor 15, connected between the source 11 and load 17, and a neutral return conductor 16, connected between the load 17 and source 11, using a differential current transformer 14. A current Ip flowing in phase conductor 15 and a return current Ir flowing in neutral conductor 16 are equal in magnitude (that is, the vector sum of Ip and Ir is zero) and no ground fault or residual current is flowing. The GFCI device 12 is configured to open contacts 13a, 13b to interrupt the current flow from source 11 to load 17 when it detects a difference in current between the phase and neutral conductors 15, 16.
FIG. 2 shows the circuit FIG. 1 but with a ground fault current Igf flowing in the grounded conductor 19 caused by a high impedance short-circuit 20 between the phase conductor 15 and the grounded enclosure 18. The value of the ground fault current Igf is a vector sum of the currents Ip, and Ir flowing in the circuit 10 and can be expressed as a phasor, having both magnitude and phase.
It is well known that for conventional electrical systems, analysis of sinusoidal AC current and voltage performance is simplified by using a phasor characterization of the sinusoids. Such phasor characterization uses complex numbers, having “real” components associated with resistive elements, and “imaginary” components associated with reactive elements. Phasors are represented alternatively in polar or rectangular form on the complex plane. The rectangular representation of a phasor for an AC signal comprises both a real component and an imaginary component. The polar representation of a phasor for an AC signal comprises both a magnitude and an angle.
A conventional phasor diagram for the ground fault current Ig of FIG. 2 is shown in FIG. 3. In FIG. 3, the reactive, imaginary current component 33 flowing through the capacitance is shown at a right angle to the resistive, real current component 31. The imaginary current component 33 is purely reactive so it does not cause heating and does not present a shock hazard. Thus, the reactive current component 33 of the ground fault current does not necessitate tripping of the GFCI device 22 (FIG. 2). In contrast, the resistive component 31 does cause heating and does present a shock hazard. Thus, only the real component 31 of the current Igf necessitates the GFCI device 22 (FIG. 2) to trip at predetermined current levels; the imaginary component 33 does not.
It will be understood that in conventional electrical systems there is generally a capacitance between the circuit conductors and ground. A relatively high capacitance-to-ground is common on single and three-phase electrical distribution systems having voltages greater than 120 volts line-to-ground and results in a characteristic capacitive current. The relatively high capacitance-to-ground can be caused by various factors such as long conductors to a load, or by phase-to-ground capacitors connected in the circuit.
One problem that occurs with conventional GFCI devices in circuits with a high capacitance-to-ground is unnecessary or nuisance tripping caused by the capacitive current to ground in the absence of a real, or resistive, ground fault condition. That is, the capacitive, or reactive, currents can exceed the predetermined ground fault current threshold of conventional GFCI devices and result in a nuisance tripping of the GFCI in the protected circuits.
At voltages above 120 VAC, and at relatively low ground fault current trip settings in the milliamp range, the reactive portion 33 of the ground fault Igf, is known to cause nuisance tripping of conventional ground fault devices 22 (FIG. 2). Such conventional devices have not been able to distinguish between the resistive (real) portion 31 of the ground fault and the reactive (imaginary) portion 33 of the ground fault current at the higher voltages.
Conventional methods of detect hazardous ground faults and distinguishing between real and imaginary parts of the ground fault current have determined a phase angle between the ground fault and phase currents, and measured a period of the ground fault alternating current and phase currents between respective zero crossings, and divided the respective time periods of the ground fault and phase currents. Additionally, the ground fault signal, and the measured period between respective zero crossings signal have been provided as one or more pulse signals to a controller.
One known shortcoming of such a conventional method, in the instance that the ground current is noisy or non-sinusoidal, the ground fault zero crossings may alter timing measurement of the conventional method, thus resulting in an incorrect zero crossing measurement. Moreover, since the controller must generate a pulse and measure pulses in order to determine the zero crossings and measure the time between zero crossings, the prior art methods can be complicated and subject to signaling errors.
For at least the reasons stated above, a need exists for an improved device and method to detect hazardous ground faults on single phase and multi-phase circuits at voltages over 120 VAC.